I linked to this article by Jack Hitt earlier, but in case you didn’t click the link, check out his conclusion:

. . . A 2011 study published in the journal Animal Cognition found that even expertly trained dogs and the most professional handlers cannot evade what is called the Clever Hans effect. In tests, dogs trained to detect explosives and drugs were sent, with their handlers, into a series of rooms to find non-existent contraband. In one room, there was a decoy that had been scented with sausage; in another, there was an unscented decoy accompanied by a sign telling the handler, falsely, that it smelled of contraband; a control room had no decoys. The investigators found, overall, that “human more than dog influences affected alert locations”: the meat decoy attracted more false alarms than anything in the control room, but the decoy with the sign prompted nearly twice as many false alerts as the…